Control system for coupling circuits



Oct. 27, 1970 J. E. Rm ETAL. 3,537,010

CONTROL SYSTEM FOR COUPLING CIRCUITS Filed July 27, 1967 JORGE E, ROZAALB/5 KURV/TS QMMQJM United States Patent 3,537,010 CONTROL SYSTEM FORCOUPLING CIRCUITS Jorge E. Roza and Aldis Kurvits, Monroe, N.Y.,assignors to General Dynamics Corporation, a corporation of DelawareFiled July 27, 1967, Ser. No. 656,513 Int. Cl. H04b 1/10 US. Cl. 325-1747 Claims ABSTRACT OF THE DISCLOSURE An automatic control system isdescribed for adjusting reactive elements in a coupling circuit disposedbetween an active element and an output network so that a maximum amountof power is transferred from the active element to the output network.The control system is adapted to continuously adjust the reactiveelements in the coupling circuit until it senses that the voltageapplied to the coupling circuit has just passed its maximum level, atwhich time the control circuit stops adjusting the reactive elements.

The present invention relates to a control system for adjusting theimpedance of a coupling circuit.

The impedance of an input device, such as an RF power amplifier, iscommonly matched to an output network, such as an antenna, by carefulvalue selection of the reactance of elements in a coupling circuit. Mostcoupling circuits employ at least two adjustable impedance elements, oneof which is for phase correction, and the other is for impedancemagnitude correction. The control systems used in this scheme have oftenbeen complex. Moreover, many of the prior control systems encounterdifiiculties by either failing to adjust the coupling circuit so that itachieves a satisfactory impedance match or require a relatively longperiod to tune the coupling circuit. In this connection, priorarrangements have had especial difficulties sensing phase angles andimpedance magnitudes when the antenna broadcasts over a broad frequencyspan of say from 2 to 76 megahertz.

In view of the foregoing, it is an object of the invention to provide animproved control system for an impedance matching coupling circuit whichrequires relatively little tuning time even though the frequency rangecovered may be appreciable.

It is a further object of the invention to provide an improved controlsystem for coupling circuits which is relatively simple, easy tomanufacture, and which has a high degree of accuracy.

In accordance with one exemplary embodiment of the invention, there isprovided a coupling circuit disposed between the output tube of atransmitter and an antenna network and which has at least two reactiveelements which are simultaneously adjusted by means of a motor. Thecontrol system employs sensing means for determining when the voltage ofthe output tube just passes through its maximum amplitude. At this time,these means stop the tuning motor so that the impedance of theadjustable elements are now set with the coupling circuit closelymatching the impedance of the tube to the antenna. Stated differently,when the output voltage from the tube just swings past a maximum, thecoupling circuit will be properly tuned.

The invention itself, both as to its organization and method ofoperation, as well as additional objects and advantages thereof willbecome more readily apparent from a reading of the following descriptionin connection with the accompanying drawing which is a diagrammaticillustration, partially in block and partially in schematic form,showing a representative control system in accordance with theinvention.

"ice

Referring to the drawing, there is shown a coupling circuit 10interposed between an RF power amplifier 11 including an active element,such as an output tube 12, and an output network which in this instanceis shown to be an antenna 13 connected in series with a coil 14.

In the circuit 10 a capacitor 1 6 is coupled across the output of thetube 12 which of course is driven by an RF signal. The circuit 10 alsoincludes an inductor element 20 connected in series with the capacitor16 and a shunt inductive element 21 across which the output voltage fordriving the antenna 13 is developed. Two capacitors 25 and 26 areprovided in the output circuit which couples the element 21 to theantenna 13. The capacitors 25 and 26 are arranged to isolate the B+voltage from the antenna 13. Both the inductive elements 20 and 21 areadjustable coils. The element 16 and element 14 may have to be switchedor changed at convenient frequency intervals.

The elements 20 and 21 are simultaneously adjusted by means of a shaftconnection with a motor 29'. The arrangement is such that initially theelements 20 and 21 are both disposed in maximum inductance positions.During adjustment with a B+ voltage impressed thereon, the motor 29drives the elements 20 and 21 to positions having progressively smallerinductance until the impedance of the output tube 12, coupling circuit10 and the output network (viz antenna 13 and coil 14) are at resonance.At resonance the B+ potential is effectively open circuited and themotor 29 is shut 01f. At that time, of course, no further adjustment ofelements 20 and 21 take place. The control circuit which recognizes whenresonance is reached will be described later in this specification.

The reactance of each of the elements 20 and 21 is arranged to have asubstantially fixed ratio over the entire range of operatingfrequencies. Because of the arrangement of the elements of the circuit10, a fixed resistive load placed across the inductance 21 will beviewed by the tube 12 to be of a higher resistance more compatible withthe operation of the tube 12.

By virtue of the voltage dividing aspects of this coupling circuit, theload resistance seen by the tube 12 will vary approximately inverselywith resonant frequency at a much lower rate than would the more usualprior art arrangements. This feature is important inasmuch as mostactive output devices are substantially aifected by variations in powerfactor but relatively insensitive to variations in the load.

Although the circuit 10 is especially suitable for use in accordancewith the present invention, it is to be understood that the moreconventional coupling networks having adjustable capacitor elements andwhich may for example take T and pi configurations may also be employedin accordance with the invention.

It has been found that when the output voltage of an active element suchas the tube 12 is for any given frequency at a maximum, the couplingcircuit 10 will at that time match the impedance of the antenna 13 tothat of the tube 12. The present invention makes use of this fact andemploys control circuitry which senses when the voltage at the output ofthe tube just passes through its maximum level and develops a signalwhich in turn causes the motor 29 to be shut oif.

Two serially connected capacitors 31 and 32 (capacitor 32 beingconnected to ground) are connected by means of a lead 30 to the plate ofthe tube 12. These two capac itors provide a voltage dividing networkand at a junction 36 between these capacitors there is connected aconventional detector network consisting of a diode 40 and twocapacitors 42 and 46 which are jointed by a choke coil 44. The voltagedeveloped across the capacitor 46 is applied as an input signal(representative, although somewhat reduced in amplitude, of the envelopeof the signal developed by the tube 12) to an operational amplifier 50which has a characteristic that it develops a rather large negativegoing signal Whenever the output of the tube 12 just swings past itsmaximum voltage level. This negative going signal triggers a flip-flop52 so that its output voltage which is applied to the base of an NPNtransistor 56, which receives operating potential via the coils of themotor 29, switches from a positive to a negative potential, with respectto ground. In response to such an input, the transistor 56 turns off,effectively open circuiting the B+ potential for the motor 29, therebyshutting off the motor 29. At this time the circuit is properly tuned.It will be understood that the motor may include conventional apparatusand controls to prevent overshooting.

Returning to the operational amplifier 50, it performs as follows: whena rising positive signal or a constant input positive voltage isapplied, little or no output voltage is generated. However, Whenever adecreasing voltage is applied, the output voltage quickly becomespositive, thereby driving the NPN transistor 67 into a shut-offcondition. By way of example only but without limitation, it has beenfound that the operational amplifier model No. ,uA702A manufactured byFairchild Semiconductor can be readily employed in accordance with theinvention. This amplifier is especially effective because it willrecognize when the input signal has just passed a maximum position butwill not be prematurely actuated by slight perturbation in the signal.

If the frequency of the output tube is to be changed, the couplingcircuit 10 may be tuned to this new frequency by first actuatingreversing circuitry such a switch connected to the field coils and showndiagrammatically by the block 60, which causes the motor 29 to be drivento a position whereby the inductance of the elements and 21 are again ata maximum value. Thereafter, the circuitry 60 resets the flip-flop 52 sothat the transistor 56 is gated on (with a positive voltage at theoutput of the F/FSZ), permitting the B voltage to start to drive themotor 29. Tuning is thereafter accomplished as described above.

While an exemplary embodiment of the invention has been described,variations thereof and modifications therein within the spirit of theinvention will undoubtedly suggest themselves to those skilled in theart. For example, although the disclosed embodiment is with reference toa coupling circuit for an RF power amplifier and an antenna, it will beunderstood that the present invention may be used to adjust couplingcircuits for a number of varieties of arrangements wherein the impedanceof an output network must be matched to that of an active element.Accordingly, the foregoing description should be taken as illustrativeand not in any limiting sense.

What is claimed is:

1. A control system for adjusting a plurality of reactance elements in acoupling circuit which is adapted to match the impedance of an outputnetwork to that of an active element comprising (a) means forcontinuously varying the reactance of said reactance elements,

(b) means for sensing the output voltage of said active elementdetermining when it reaches a maximum and then generating a controlsignal, and

(c) means responsive to said control signal for stopping the adjustmentof said reactance elements, whereby said impedances of said activeelement and said output network are substantially matched.

2. The invention as set forth in claim 1 wherein said continuousadustment means is a motor.

3. The invention as set forth in claim 1 wherein said sensing meansincludes a flip-flop.

4. The invention as set forth in claim 2 wherein said active element isa tube embodied in an RF amplifier, said output network includes anantenna, and wherein said voltage sensing means comprises means coupledacross the output of said tube for generating a DC. voltagerepresentative of the amplitude of the signal produced by said tube,operational amplifiers means connected to said DC. voltage generatingmeans for producing said control signal when said output voltage swingspast maximum amplitude, and means including a flip-flop responsive tosaid control signal for preventing said motor from adjusting saidreactive elements.

5. The invention as set forth in claim 4 wherein said DC. voltagegenerating means comprises a voltage dividing network connected to theoutput of said tube, and a detector network electrically connectedbetween said operational amplifier means and said voltage dividingnetwork.

6. The invention as set forth in claim 5 wherein said coupling circuitcomprises a capacitor connected across the output of said tube, a firstreactance element having an adjustable inductance, and a secondreactance element having an adjustable inductance, said capacitor andfirst and second reactance elements being electrically coupled, saidmotor being adapted to simultaneously vary the inductance of each ofsaid reactance elements.

7. The invention as set forth in claim 6 wherein an input voltage forsaid antenna is developed across said second reactance element.

References Cited RICHARD MURRAY, Primary Examiner R. S. BELL, AssistantExaminer US. Cl. X.R.

